1. Field of the Invention
The present invention relates to a contour measuring instrument of the type which takes measurement of the contour of an object to be measured by tracing its outer surface with a stylus secured to one end of an arm.
2. Description of the Prior Art
In general, in a contour measuring instrument, a stylus to be brought into contact with the outer surface of an object to be measured is secured to one end of an arm, the other end portion of this arm is rotationally supported by an arm support member, the center of gravity of the arm is held with the side of stylus being slightly heavier, so that the stylus can constantly be in abutting contact with the outer surface of the object to be measured. In measuring the contour of the outer surface of the object by use of the contour measuring instrument of this type, the arm support member is moved in the axial direction of the arm, the stylus is displaced in accordance with the contour of the outer surface of the object to be measured due to the movement of this arm support member so as to rotate the arm about its fulcrum where the arm is pivoted, the rotation of the arm is detected by use of contour signal detecting means such as a differential transformer, and the signal detected by this contour signal detecting means is combined with a signal for feeding arm support member to carry out the measurement of the contour of the object.
However, with such a conventional contour measuring instrument commonly used, a problem arises in that, when the stylus falls into a deep groove or the like, the stylus may be broken and the object to be measured may be damaged. More specifically, since no damage preventing mechanism for protecting the stylus and the like is provided in the conventional instrument, when the stylus falls into the deep groove or the like, the arm support member continues to be moved, and the stylus is brought into abutting contact with the wall surface of the groove under a strong pressing force, thus resulting in breakage of the stylus.
Furthermore, with such a conventional contour measuring instrument commonly used, due to rotational movement about a fulcrum where the arm is pivoted, the stylus follows circular arcs in motion, thus moving not only in the vertical direction but also in the feeding direction of the arm support member. Consequently, the movements in this feeding direction may form errors in measurement (errors in circular arcs), and, when there are considerable irregularities on the outer surface of the object to be measured, the errors may increase accordingly, hence rendering accurate measurement of the contour of the object with the stylus impracticable.
To overcome this problem, there has been proposed a mechanism wherein one end of a link bar is rotatably mounted on an end portion of an arm opposite to the side, to which a stylus is secured, through a cross spring, and the other end of the link bar is supported on an arm support member (slide plate) through a cross spring, whereby, when the arm is rotated, the link bar is rotated about the point supported on the arm support member, and due to this rotation, the arm is moved in a direction of correcting errors in the circular arcs, so that, finally, the tip end of the stylus can move substantially vertically, thereby enabling to avoid the occurrence of errors in the circular arcs, or a mechanism similar to the above, wherein parallel links are used in place of the link bar described above (Refer to Japanese Patent Application "Kokai" (Laid-Open) No. 53352/78).
However, in the conventional mechanism as described above, the link bar or parallel links for supporting the arm are required, and moreover, there has been such a disadvantage that the arm is not necessarily provided at the most preferable position, so that a force of an excessively high value may act upon the arm.
Further, in the conventional contour measuring instrument, a differential transformer as contour signal detecting means is mounted at a position spaced apart a predetermined distance from a plane incorporating therein the center of rotation of the arm and being in parallel to the feeding direction of an arm support member (hereinafter referred to as a "reference plane"). More specifically, a core (magnet) of the differential transformer is projectingly provided in a direction perpendicular to the arm, and a coil of the differential transformer is fixed on the arm support member in a manner to include the core at the center thereof, whereby the center position of the core and coil in the axial direction thereof is necessarily spaced apart a predetermined distance from the abovedescribed reference plane incorporating therein the center of rotation of the arm. With this arrangement, there is such a disadvantage that the inclination value of the core in the coil is different depending on the movement of the arm, whether the arm moves upward above the reference plane or downward below the reference plane, thereby causing errors due to the difference. There has been developed such a mechanism wherein the core is secured to the arm not directly but through parallel links, so that, when the arm is rotated, the core can vertically move without being inclined in the coil. With such a contour measuring instrument, errors due to the direction of rotation of the arm can be prevented, however, such disadvantages are presented that the resultant complex construction results in increased costs and accumulated errors due to looseness and the like of connecting portions of the respective parts amount comparatively high due to the complex construction.